Problem: If $x \bigtriangleup y = x(y-4)$ and $x \boxdot y = 4x^{2}+y^{2}$, find $(-5 \boxdot 6) \bigtriangleup 4$.
Explanation: First, find $-5 \boxdot 6$ $ -5 \boxdot 6 = 4(-5)^{2}+6^{2}$ $ \hphantom{-5 \boxdot 6} = 136$ Now, find $136 \bigtriangleup 4$ $ 136 \bigtriangleup 4 = 136(4-4)$ $ \hphantom{136 \bigtriangleup 4} = 0$.